Theorem 1 ([1]}) 
Let \(G\)  be a graph, let \(X\)  be a nonempty proper subset of \(V(G)\) , and let \(Y=V(G)\setminus X\) . Suppose that all the arcs of \([X,Y]\)  have distinct labels and that the arcs of \(G/X\)  and \(G/Y\)  corresponding to the arcs of \([X,Y]\)  are the only synchronising arcs of \(G/X\)  and \(G/Y\) .
If \(S^{\prime }(G)\subseteq X\)  and \([X,Y]\)  has no backward arcs, then
\(G\cong G/YG/X\) .

